Diffusion behavior and reliability in bonds between Au-Ag alloy and aluminum

The replacement of conventional pure Au bonding wires by highly-alloyed Au-Ag ones in LSI semiconductors can reduce significantly material cost. However, Au-Ag wires have not yet been used due to a failure problem at the bonding points between the Au-Ag balls and Al thin film pads. Therefore, to apply Au-Ag wires to LSI products, the improvement of the thermal reliability at the bonding points between Au-Ag/Al is very important. The thermal reliability of the Au-Ag/Al bonds depends on the Ag concentration (C_Ag) in Au wires. A certain range of C_Ag showing good bond reliability was found. Ball bonds of Au-14 at%Ag produced a significant degradation after annealing. On the contrary, bonds of Au-24 at%Ag provided as good a reliability as the commercial pure Au wire after annealing at 473 K for 1000 hr. Thus, the optimization of the Ag concentration in the Au-Ag alloy wires was significantly effective in improving the bond reliability. Diffusion behavior at the bond interface of the Au-Ag/Al was different from that of pure Au/Al. It was characteristic that AuAl₂ grew predominantly in the interface of the Au-Ag/Al. The formation of intermetallic compounds varies with C_Ag. The dependence of the intermetallic compounds (Au₅Al₂, AuAl₂, Ag₂Al) on the Ag concentration dominated the bond reliability.